1. Field of the Invention
The present invention relates to a telescopic multi-stage boom used in a crane which shifts a hook up and down to lift a weight load, and used in an elevating device which shifts an elevating stand up and down above a vehicle body to lift a person or material. More specifically, the present invention relates to a reinforcement mechanism of the multi-stage boom, the mechanism being provided with flexible cables for supporting a part of a weight load applied to a top end portion of the multi-stage boom so as to avoid the multi-stage boom from bending.
2. Prior Art
There is known and frequently used a lifting mechanism including a crane mounted on a vehicle such as a truck and operable to lift a weight load so as to load and unload cargos.
There is also known and frequently used an elevating working station vehicle provided with an elevating station for use in assembling, painting and repairing work at high places such as highways and building constructions. The elevating station may accommodate workers and materials to transport them between the ground and elevated level.
These conventional crane and elevating working station vehicles are often provided with a multi-stage boom comprised of booms having different outer diameters and each being telescopically inserted into the other to undergo extension and contraction in their lengthwise direction.
This multi-stage boom has a very short total length in the most contracted state and a very long total length in the most extended state, and therefore is frequently installed on a crane vehicle and elevating or high working station vehicle which have a limited total length of vehicle body, because the multi-stage boom has the advantage that the multi-stage boom can be most contracted in compact size during the travel of vehicles.
However, since the multi-stage boom mechanism is comprised of a plurality of booms having different outer diameters and being telescopically connected to each other, there are many sliding contact portions in the mechanism. For this reason, there are drawbacks such as excessive play may be produced during the course of long time operation and such as the top end portion of the boom mechanism may swing up and down when extended longer or when lifting a heavier weight load. Consequently, in order to lift a heavy weight load, each boom must be physically strengthened, resulting in increase of the total weight of the multi-stage boom. Further, since each boom is made of metal, when a heavy weight load is applied to the top end of the multi-stage boom, the multi-stage boom is bent due to the heavy weight load and deformed in arch shape. Therefore, when the multi-stage boom lifts a heavy weight load, the multi-stage boom sometimes cannot be smoothly slided in the lengthwise direction thereof.
As described above, while the conventional multi-stage boom has a very short total length when most contracted, it has the drawback that each boom must be physically strengthened and made in rigid shape in order to withstand against an applied heavy weight load, thereby causing increase in the total weight. Further, each boom may be bent when applied with a weight load. In such case, sliding contact portion between adjacent booms must be accurately shaped to enable the smooth sliding even under bent state and therefore very high accuracy is required in shaping each boom.